A polyolefin microporous membrane is used, for example, as a filter, a separator for a fuel cell, or a separator for a capacitor. In particular, the membrane is suitably used as a separator for a lithium ion battery widely employed for a notebook personal computer, a cell-phone, a digital camera or the like. The reason therefor is that the polyolefin microporous membrane is a membrane having excellent mechanical strength and shutdown properties. Furthermore, the lithium ion secondary battery is being developed toward achieving higher energy density, higher capacity and higher output, and in association therewith, higher properties are required for the separator.
The production method of a polyolefin microporous membrane includes a method of blending a plasticizer or an inorganic filler with a resin composition and extracting the plasticizer or inorganic filler before membrane production or after stretching (wet process), and a method of opening pores by utilizing a crystal interface of a resin composition or an interface of an inorganic filler and a resin composition, without blending a plasticizer (dry process).
The dry process does not require a plasticizer extraction step, a drying step or the like and is, therefore, excellent in economical efficiency, but at the same time has a problem that, for example, the mechanical strength is low and since uniform pores are difficult to obtain, the quality is liable to be uneven. On the other hand, the wet process includes a large number of steps compared with the dry process and has poor economical efficiency but at the same time, is advantageous in that uniform pores can be obtained and the mechanical strength is excellent.
The wet process includes two techniques, i.e., a method of extracting a plasticizer before stretching and a method of extracting a plasticizer after stretching. As for the method of extracting a plasticizer before stretching, JP-A-2010-24463 discloses a method of melting and kneading a polyolefin and a plasticizer to produce a sheet and extracting part of the plasticizer before stretching. In that technique, since the plasticizer is removed before stretching, compared with the case of stretching the sheet in a plasticizer-containing state, stretching tension is high and stretching may not be easily performed. Also, JP-A-2012-144649 discloses a method of melting/kneading a polyolefin, a plasticizer and an inorganic particle to produce a sheet, extracting the plasticizer and the inorganic filler with use of a solvent, and performing stretching. In that membrane production method, since a plasticizer is not contained at the time of stretching, the stretching tension is high. In addition, pores are formed using an inorganic filler, giving rise to a problem that the pores are coarsened and non-uniformized. Accordingly, when stretching is performed at a high stretch ratio to obtain high mechanical strength, it is disadvantageously difficult to stably produce the membrane.
In the method of extracting a plasticizer after stretching, the polyolefin is softened by the plasticizer, and this facilitates molding, provides excellent process stability and enables formation of uniform pores. Because of these advantages, the method of extracting a plasticizer after stretching is commonly used. The stretching method includes simultaneous biaxial stretching of simultaneously conducting longitudinal stretching (hereinafter, MD stretching) and transverse stretching (hereinafter, TD stretching), uniaxial stretching of subjecting a gel-like molded product before extraction of a plasticizer to stretching at least in one direction, and sequential biaxial stretching of performing TD stretching after MD stretching.
JP-A-2010-106071 states that simultaneous biaxial stretching is preferably used from the viewpoint of enhancing the puncture strength of a polyolefin microporous membrane and providing a uniform membrane thickness. However, as described in JP-A-2009-91461, in applying a high stretch ratio, since a sheet before stretching is thick, simultaneous stretching is liable to be associated with a phenomenon of separation of the sheet from a chuck, and the production stability may decrease. Furthermore, the simultaneous biaxial stretching method has a problem that due to an equipment issue, adjustment of MD and TD stretch ratios is difficult and the adjustable range of film physical properties is narrow.
JP-A-2009-91461 and JP-A-2011-210573 describe sequential stretching. The stretch ratio, stretching speed, stretching temperature and the like in MD and TD can be appropriately adjusted, and a wide range of products can be produced. In addition, stretching is conducted separately in MD and TD, and an orientation can thereby be efficiently created, which is advantageous in that, for example, compared with simultaneous stretching, high strength is likely to be obtained even at the same areal stretch ratio.
It has been conventionally known that the overall strength of a separator film is enhanced by the addition of an ultrahigh molecular weight polyolefin (hereinafter, UHMwPE), and JP-A-2011-201949 describes a sequential stretching method using UHMwPE and adopting a method of extracting a plasticizer before stretching. However, JP '949 discloses that the degree of orientation is controlled by using UHMwPE having a large molecular weight to achieve excellent heat resistance, SD temperature and permeability. Although UHMwPE is added, good strength is not obtained, and the shrinkage ratio is increased. In addition, since an ultrahigh molecular weight polyolefin is added, the pressures of an extruder and a spinneret are high and when the discharge rate of a resin is raised to increase the thickness of a sheet before stretching for obtaining a higher MD stretch ratio, there arises a problem that, for example, the membrane production is difficult due to an excessive increase in the pressure. Also, a problem of incapability of ensuring stretchability or uniformity still remains.
JP-T-2013-530261 describes a method not using UHMwPE as the production method ensuring excellent extrusion, kneading and stretching properties, which are the challenge above. Adoption of this method makes it possible to realize high discharge by virtue of low viscosity compared with the UHMwPE addition system, prevent a problem that membrane production is difficult due to an excessive increase in the stretching tension, and obtain a microporous membrane with excellent productivity. In addition, a microporous membrane having higher strength and permeability than in JP-A-2011-201949 is obtained. However, heat resistance that has been conventionally unachievable only with HDPE is achieved by the addition of UHMwPE and, to serve customer needs diversified accompanying the trend toward higher energy density, higher capacity and higher output, UHMwPE must be added.
As described above, the sequential wet stretching system is advantageous in that various products can be produced to meet diversified customer needs with less mechanical restriction and the productivity is excellent, but on the other hand, has a drawback of difficulty in terms of high discharge keeping up with an increase in the degree of freedom of stretch ratio, productivity including appearance improvement, and UHMwPE addition.
It could therefore be helpful to provide a polyolefin microporous membrane having excellent strength, permeability and heat resistance, which is obtained by using UHMwPE and employing a sequential stretching system, and a production method of the microporous membrane.